Abstract

Over 260,000 sq km (96,525 sq m) of the outer continental margin of southwest Africa have been affected by Neogene slump structures. They occur along the whole of the margin between the eastern Walvis Ridge and the southern tip of the African continent (about 200S to 370S) North of about 300S, four discrete slump zones have been recognized, but to the south, the margin has been cut by several (at least four) coalescing features which form an almost continuous zone over 850 km (527 mi) long. Three seismic profiles and interpretative sections are shown to illustrate the geometry and associated structures of these slumps (Figure 1) - two from the large feature (Chamais Slump) immediately north of 300S (sections A and B) off southern Namibia, and one from the complex zone southwest of Cape Town (section C). Further details of the geological setting and overall geometry of these and similar large allochthonous structures on the nearby southeast continental margin can be found in Emery et al (1975), Dingle (1977, 1979), Bolli et al (1978), Summerhayes, Bornhold, and Embley (1979), and Dingle, Seisser, and Newton (1983). Figure 2 diagrammatically shows the main morphological features that have been used by various workers to identify large slumped masses from seismic profiles.

In the following account of the seismic profiles A-C, vertical scales are shown in seconds of traveltime (two-way time), which in the interpretive sections have been converted to water depths at 1,500 m/sec (4,921 ft/sec) (meters on the left, fathoms on the right). In the discussion, thicknesses of strata are quoted in meters, based on a nominal P-wave velocity of 2,000 m/sec (6,562 ft/sec) in the sediment. On the interpretive sections, thick lines indicate glide planes, and opposed arrows show directions of relative movement along some of the larger glide planes. Slope inclinations are calculated from the horizontal, and quoted as a ratio (for example, 1 in 10 =1:10).

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Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.

Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.

Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.

The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.

The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.